1. Field of the Invention
The present invention relates to a method of producing an oriented silicon steel sheet having a very high magnetic flux density and, more particularly, to a novel way of effectively overcoming the loss of AlN in the surface layer of the steel.
It has been found that the AlN tends to be consumed during normalizing annealing or intermediate annealing, causing important disadvantages. The AlN loss serves to cause deterioration of the magnetic flux density which can be caused by a reduction in the thickness of the product sheet, and hence the AlN loss serves to deteriorate the desired high magnetic flux density in the steel regardless of the sheet thickness.
2. Description of the Related Art
Oriented silicon steel sheet is mainly used as an iron-core material for transformers, and is required to possess the magnetic characteristics of exhibiting high magnetic flux density and small core loss.
In recent years, advances in production technology have made it possible to produce, on an industrial scale, products possessing excellent magnetic characteristics of the above-described kind. For example, in the case of steel sheets having a sheet thickness of 0.23 mm, it has been made possible to produce products having a magnetic flux density B.sub.8 (value at a magnetizing force of 800A/m) of 1.92T and an iron loss characteristic W.sub.17/50 (value at the time of maximum magnetization at 50 Hz and 1.7T) of 0.90W/kg.
Materials having such excellent magnetic characteristics are comprised of a crystalline texture in which the &lt;001&gt; orientation, serving as the axis of easy magnetization, is highly aligned with the direction in which the steel sheet has been rolled. Such a texture is formed by a phenomenon known as secondary recrystallization during final finish annealing among the production processes of an oriented silicon steel sheet. In the secondary recrystallization, those crystal grains having the (110) [001] orientation are preferentially grown to a giant size. It is known that the fundamental requirements for sufficient growth of secondary recrystallized grains having the (110) [001] orientation are: (i) the existence of an inhibitor for restraining, in the process of secondary recrystallization, the growth of crystal grains having unpreferred orientations which are other than the (110) [001] orientation; and (ii) the formation of a primary recrystallization texture suitable for sufficient development of those secondary recrystallized grains having the (110) [001] orientation.
In general, fine precipitates of MnS, MnSe, AlN and the like are used as the inhibitor. Also, a method is known, in which, in addition to a precipitate (such as above), a grain boundary segregation type element (such as Sb or Sn) is used to strengthen the effect of the inhibitor.
Conventionally, a method in which MnS or MnSe is used as the main inhibitor has been regarded as advantageous for the purpose of reducing iron loss because the method achieves a small size of secondary recrystallized grains. Recently, however, it has become possible, by adopting such a method as a laser radiation method or a plasma jet method, to introduce artificial grain boundaries, and thus to form fine magnetic domains. As a result, the achievement of a small size of secondary recrystallized grains has become less regarded as advantageous than before and, instead, it has become more important to increase the magnetic flux density.
Methods for obtaining oriented silicon steel sheets having high magnetic flux densities have long been known. For example, it is known from Japanese patent publication No. 46-23820 that such a steel sheet can be produced by: (1) causing a steel to contain AlN as an inhibitor component; (2) effecting rapid cooling as cooling in the annealing before the final cold rolling, thereby allowing AlN to precipitate; and (3) employing a high rolling reduction of 80 to 95% in the final cold rolling.
However, the above known method has the disadvantage that, when the sheet thickness of the product is reduced, the magnetic flux density is greatly deteriorated. With this method, therefore, it has been very difficult to stably produce products which have a sheet thickness of not more than 0.25 mm and which have a B.sub.8 value of not less than 1.94T, despite such products having recently been desired.
In view of the above-described circumstances, the present inventors have previously filed an application for the art described in Japanese patent laid-open No. 2-115319. This art is based on the finding that, if Sb is added to an oriented silicon steel including AlN as the main inhibitor, and, simultaneously, if the final finish annealing method is improved, it is possible to obtain a material having a very high magnetic flux density even when the steel sheet has a small finish sheet-thickness.
However, even with the method according to the present inventors' previous proposal, it has not always been easy to stably produce a material having a high magnetic flux density on an industrial scale.
It has been found that, if Sb is present, as described above, a problem arises in an industrial-scale production in that the secondary recrystallization does not take place. This makes it very difficult to stably obtain a material having a high magnetic flux density.
Specifically, the following case was often observed: even when a material obtained by processing a hot rolled steel sheet in a laboratory exhibited a very high magnetic flux density, a coil of the same material subjected to the same type of processing on an industrial scale failed to exhibit a high magnetic flux density, and also failed to undergo secondary recrystallization.
In order to determine the cause of the failure, the present inventors collected samples obtained from each of the processes, and examined the samples. As a result, the cause was determined on the basis of the fact that no precipitation of AlN, the main inhibitor, was detected in the surface layer portion of the steel sheet after, for instance, normalizing annealing and intermediate annealing. That is, it was discovered that dissipation of AlN caused the inhibiting ability of the steel sheet surface layer to be reduced, and thus permitted normal grain growth to take place in the final finish annealing, whereby secondary recrystallization failure occurred, and that the above phenomenon was the cause of failure in an industrial-scale production.
The phenomenon in which AlN in the surface layer of the steel sheet is consumed by normalizing annealing, intermediate annealing or a like process, occurs also with respect to steel containing no Sb. However, the phenomenon does not lead to any particularly serious consequence when the steel is the non-Sb type. We believe this is because, during final finish annealing, the renitriding of the steel sheet surface layer takes place before secondary recrystallization, whereby an AlN precipitate is again generated in the surface layer portion of the steel.
Specifically, during the final finish annealing (box annealing), the steel sheet being processed is exposed for a relatively long period of time to a nitrogen atmosphere in a stage prior to the start of the secondary recrystallization (within a temperature range below 900.degree. C.). This exposure allows an excessive amount of Al contained in the steel to diffuse to the surface layer portion, and to combine with nitrogen diffusing from the surface of the steel sheet, thereby allowing AlN to reprecipitate. By virtue of the reprecipitation of AlN, the inhibiting ability of the surface layer of the steel sheet, which has been temporarily lost, is recovered in a timely manner before the start of the secondary recrystallization. For this reason, the phenomenon of AlN consumption has not been revealed.
However, in the case of steel containing Sb, the mechanism of recovering the inhibiting ability of the surface layer does not work. This is believed to be because Sb, which segregates on the steel sheet surface, acts to restrain nitriding, thereby making it very difficult for AlN, once consumed, to be reprecipitated and thus restored.
Regarding the art of strengthening the inhibiting ability of the surface layer of a steel sheet, Japanese patent publication No. 50-19489 discloses the art of employing nitrogen as an atmosphere during annealing of an oriented silicon steel containing Al, and nitriding the surface of the steel sheet, thereby precipitating AlN. This art has been tried by the present inventors. However, it was confirmed that, with respect to a steel sheet containing Sb, the nitriding was restrained by the above-discussed phenomenon, and it was difficult to improve the magnetic characteristics of the product.